U.S. Air Force Staff Sgt. Richard Clark, 633rd Security Forces base defense operations center controller, responds to an active shooter during an exercise at Langley Air Force Base, Va., Dec. 11, 2014. The exercise was initiated with SFS receiving a call of an active shooter at the Shellbank Fitness Center, in which point an eight-Airmen fire team assembled and responded. (U.S. Air Force photo by Senior Airman Kayla Newman/Released)